Methods of determining a thermal conductivity and a thermal diffusivity of a material include direct current methods, laser flash analysis, or other methods. Direct current methods may include disposing a sample of an unknown thermal conductivity between a heat source and a heat sink, contacting the sample with a standard material (e.g., a so-called “puck”), and inputting a known amount of heat into the sample with the heat source. The thermal conductivity of the sample is proportional to the power input into the sample, the temperature difference between two thermocouples placed across the sample, and the ratio of the length between the thermocouples to the cross-sectional area of the sample
      (          k      =                        P                      Δ            ⁢                                                  ⁢            T                          ⁢                  (                      L            A                    )                      )    ,wherein k is the thermal conductivity, P is the power input into the system, L is the length between the thermocouples, and A is the cross-sectional area of the sample through which the heat flows. However, an accurate measurement of the thermal conductivity requires adequately flowing heat through the sample. Accordingly, the sample must be physically coupled to the heat source and the heat sink. Improperly coupling the sample to the heat source and the heat sink may introduce undesired uncertainty in the measured thermal conductivity of the sample. In addition, it may be difficult to obtain accurate thermal conductivity measurements of thin film samples since coupling methods may damage such samples. Further, as the size of the sample decreases, the accuracy of the temperature measurements to determine the difference in temperatures between the thermocouples must increase.
Laser flash analysis, on the other hand, includes a long pulse laser and an infrared detector. The long pulse laser heats a sample and the infrared detector measures the temperature on the back side of the sample by measuring the black body radiation of the sample. Thus, laser flash analysis requires access to both sides of a sample. Laser flash analysis requires a separate measurement of the specific heat of the sample to determine the thermal conductivity thereof. Use of a laser flash analysis machine requires a particular sample size (e.g., diameter and thickness), depending on the conductivity range of the sample. In addition, smaller samples may require optical focusing of the long pulse laser used to heat the sample. Furthermore, laser flash methods often assume that the sample exhibits only one-dimensional heat flow (through the thickness of the sample). As such, the sample must meet particular thickness and diameter requirements for accurate measurement of thermal conductivity.
Thermal conductivity is an important property of many materials, for various applications. For example, in nuclear fuel systems, the thermal conductivity of nuclear fuel is related to energy conversion efficiency as well as to reactor safety. However, over the lifetime of nuclear fuel, the thermal conductivity thereof may degrade due to changes in material microstructure caused by neutron irradiation. In addition, the microstructure of the nuclear fuel may change over distances as short as a few millimeters from the center of a fuel element to the sidewalls or rim of the fuel element. It is often desired to know or estimate the thermal conductivity and the thermal diffusivity of a material to estimate or predict performance or other properties of the material.